The invention relates generally to voltage controlled oscillators, and, more particularly, to a digital voltage controlled oscillator.
Voltage controlled oscillators (xe2x80x9cVCOsxe2x80x9d) are used in many applications to produce an oscillating signal having a frequency defined by an input voltage. Generally speaking, there are two types of voltage controlled oscillators, namely, non-crystal voltage controlled oscillators (hereinafter xe2x80x9cnon-crystal VCOsxe2x80x9d) and crystal voltage controlled oscillators (hereinafter xe2x80x9cVCXOsxe2x80x9d). Both types of known VCOs suffer from certain disadvantages. For example, non-crystal VCOs usually exhibit poor stability characteristics (e.g., the frequency of the oscillating output signal drifts), and typically include an excessively large frequency range (e.g., 2:1 to 10:1 frequency adjust). On the other hand, while VCXOs are very stable, they tend to be expensive and have small frequency ranges. VCXOs are also usually limited to outputting a square wave. The square wave output is passed through a low pass filter to produce a sinusoidal wave, when desired.
In accordance with an aspect of the invention, a digital voltage controlled oscillator (DVCO) is provided. The DVCO includes an input for receiving input signals representative of a desired frequency. It also includes a pulse generator for generating pulses and a logic circuit in communication with the input and the pulse generator. The logic circuit is adapted to develop an oscillating signal having a predefined waveform and the desired frequency by controlling the energy contained in the pulses output by the pulse generator. The DVCO also includes a capacitor in communication with the pulse generator. The capacitor is charged by the pulses to a voltage that generally varies in accordance with the predefined waveform and the desired frequency.
In accordance with another aspect of the invention, a digital voltage controlled oscillator is provided. The DVCO includes an input for receiving voltage signals, and a duty cycle register for temporarily storing duty cycle values. The DVCO also includes a PWM generator in communication with the duty cycle register for developing an output pulse having a duty cycle specified by the duty cycle in the duty cycle register, and a waveform table storing a plurality of duty cycle values. Additionally, the DVCO is provided with a waveform controller in communication with the input for setting a frequency in accordance with the voltage signals received by the input. The waveform controller is in communication with the waveform table for sequentially writing duty cycle values to the duty cycle register at a rate defined by the frequency. The DVCO is further provided with a low pass filter in communication with the pulse generator for developing an oscillating waveform having a shape defined by the duty cycle values stored in the waveform table and a frequency defined by the voltage signals received by the input.
In some embodiments, the DVCO also includes a waveform corrector cooperating with the waveform controller to selectively adjust the rate at which the duty cycle values are written to the duty cycle register. In some such embodiments, the waveform corrector varies the rate at substantially evenly spaced time intervals within a waveform cycle to limit distortion of the oscillating waveform.
In accordance with still another aspect of the invention, a method is provided for providing a voltage controlled oscillating signal with a programmed logic circuit comprising a PWM generator. The method includes the step of creating a table of duty cycle values. Each of the duty cycle values corresponds to a predefined point on a predetermined waveform associated with the oscillating signal. The method also includes the steps of:
receiving a voltage signal indicative of a desired frequency for the oscillation signal; developing a first period from the voltage signal; sequentially delivering the duty cycle values to the PWM generator at a rate defined by the first period; and sequentially outputting pulses with the PWM generator to a capacitor. The pulses have duty cycles corresponding to the duty cycle values. The voltage across the capacitor defines the oscillating signal.
In some embodiments, the method further includes the step of selectively adjusting the rate at which the duty cycle values are delivered to the PWM generator.
Other features and advantages are inherent in the apparatus claimed and disclosed or will become apparent to those skilled in the art from the following detailed description and its accompanying drawings.